1. Field of the Invention
The present invention relates to an improved process for the direct preparation of beta-methylthiopropionaldehyde by reacting methylmercaptan with a gaseous mixture which contains acrolein and which results from the catalytic, air oxidation of propylene in the presence of water.
2. Description of the Prior Art
In copending allowed application, Ser. No. 696,432, filed June 15, 1976, now U.S. Pat. No. 4,225,516, and assigned to the assignee hereof, there is featured a process for the preparation of beta-methylthiopropionaldehyde from methylmercaptan and a gaseous mixture comprising acrolein resulting from the catalytic oxidation of propylene. In accordance with this process, in a first stage, the crude gaseous mixture emanating from the oxidation reactor is charged into an absorption column wherein the acrylic acid present in the gaseous mixture is removed; in a second stage, the gaseous mixture no longer containing acrylic acid is condensed so as to remove the water present therein. After such removal of both the acrylic acid and the water, the gaseous mixture containing acrolein absorbed in beta-methylthiopropionaldehyde is reacted with methylmercaptan to provide beta-methylthiopropionaldehyde.
The main disadvantage of this type of process is that, during the condensation operation for the purpose of removing the water present in the gaseous mixture, it is physically impossible not to condense, at the same time, some of the acrolein present in the gaseous mixture. Thus, such operation affords an aqueous solution containing from 10 to 20% of the acrolein formed during the oxidation of propylene.
In order to avoid losing this appreciable amount of acrolein, it becomes necessary to separate it from the aqueous solution, preferably by distillation, and to reintroduce or recycle same into the third stage mentioned above, namely, the stage involving the reaction of the methylmercaptan with the acrolein.
This separation and reintroduction or recycling entail an isolation process and hence the collection of a mass of virtually pure acrolein, and this flies in the face of the very spirit of the process, the object of which being to prepare beta-methylthiopropionaldehyde without the necessity for an intermediate isolation of the acrolein. In fact, this is of great practical importance, in particular on an industrial scale, because it is well known to those skilled in the art that acrolein is a toxic material, the handling of which involves numerous risks and which has been the cause of numerous serious environmental accidents.
Furthermore, the recycling of a stream of concentrated acrolein together with the dilute gas stream, in the step involving reaction with the methylmercaptan, is complicated by the fact that, according to the aforementioned patent application, it is imperative to conduct the reaction within strict limits vis-a-vis the acrolein/methylmercaptan stoichiometry; in fact, the hemithioacetal concentration within the reaction mixture should be permanently maintained between 0 and 1%, the value O being excluded.